H3K27 trimethylation adds three methyl groups to lysine 27 of histone H3, conventionally H3K27me3. It is associated with Polycomb-linked repression of developmental genes in many eukaryotes. Deposition and removal help regulate the persistence of a chromatin state. A broad marked domain describes a molecular state; demonstrating that the mark is required for repression at a particular gene needs a perturbation that separates it from associated factors. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 7, Chromatin Structure and Gene Regulation: Polycomb-associated repression
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